This is a phase II/III study of the use of recombinant human factor IX (rhFIX) in the treatment and prevention of bleeding in severe or moderate hemophilia B patients who have previously received blood products. The objectives are to assess the safety and efficacy of rhFIX administered by IV infusion in patients with severe or moderate hemophilia B during transfusion, or prevention of bleeding episodes, to estimate PK parameters of rhFIX for each patient and to monitor for any changes in PK parameters after repeated use of rhFIX.